Frédéric Oehl Gérard Cécé Olga Kouchnarenko David Sinclair Automatic Approximation for the Verification of Cryptographic Protocols. 33-48 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2629&spage=33 2002 conf/fasec/2002 FASec db/conf/fasec/fasec2002.html#OehlCKS02